1. Technical Field
The present invention generally relates to an automotive alternator system comprising an alternator having significantly enhanced power capability above idle in comparison with conventional alternators of the same size and which are designed with the same power capability of idle. The technology is particularly applicable to Lundell style alternators and dual voltage systems.
2. Related Art
Electronic valve actuation systems require only modest power at low speeds with power growing into the multiple kilowatt range at high engine speeds. One approach to providing the necessary system power at idle and the much higher valve power at high speeds is the classical Lundell Technology. A classical Lundell alternator, if optimized for the more modest power required at idle, would only lead to about 1.6 times this power at very high speeds. Even if the system is de-tuned from the optimum power at idle and is designed only to provide the required power at idle and the required power at high speed, this approach would lead to a much larger alternator exhibiting excess inertia, belt slip and difficulty of high speed operation. Such a system is not practical for providing the high power required by electronic valve actuation systems.
Typically, an electronic de-icing function for vehicle windshields requires a significant amount of power to effect rapid ice removal before the vehicle is driven. The vehicle alternator system could potentially provide this de-icing power as well as the other vehicle functions. One such state-of-the-art windshield design is the thin metallic film windshield. In order to achieve rapid ice removal for this windshield, the vehicle de-icing system would require power between about 1,200 and 1,500 watts for a 3-5 minute period. Furthermore, the de-icing system may require a relatively larger operating voltage, e.g. 42 volts to meet constraints associated with the windshield technology. The alternator system would have to provide output power sufficient to enable proper operation of the de-icing function, at the required voltage, while preferably also supplying other vehicle power functions. One possible solution to this problem of power generation is to construct conventional alternators of relatively larger size which can supply the necessary power at idle. Issues of size, inertia, high low speed torque, belt slip and required high-speed operation combine to make this a very difficult alternator design issue. Furthermore, space within the vehicle engine compartments is already at a premium.
Another possibility is to operate a conventional Lundell alternator at 2-3 times the normal idle speed during the deicing function. However a Lundell system designed to maximize power at idle would only provide a 40-50% increase in power at these elevated speeds which is still not sufficient to avoid a substantial increase in the size of the alternator. There are also power benefits through use of an initially cold alternator, but the system would still fall short and require a substantially larger alternator.
What is needed is an alternator system that is capable of providing a proportional increase in alternator power as speed increases without necessitating an increase in the physical size of the alternator.
Conventional Lundell (or other) style alternators are often designed for maximum power (current) at normal idle to achieve as much power as possible at their lowest operating frequency. In this condition, the alternator would be operating at maximum field current and would be delivering maximum current at the rated voltage. In such alternator configurations, maximum current would increase (at constant output voltage) as speed increase beyond normal idle, in accordance with a characteristic curve, the maximum current at double idle being about 1.4 times the idle current increasing to about 1.6 times the idle current at very high speeds. It should be noted here that a simple circuit model for an alternator phase winding consists of an AC voltage source in series with the winding impedance. The amplitude of the AC voltage source is proportional to rotor flux and machine frequency with each phase voltage differing by 120 degrees. The machine impedance per phase can be represented by an inductor in series with the winding resistance. Even at idle where the inductive impedance is lowest, the inductive impedance is typically much greater than the winding resistance. This inductive effect, even at idle, is the major contributor to current limitation in the machine. When machine speed increases at full flux, the amplitude of the internal phase voltage sources also increase proportionately with frequency but the inductive impedance also increases proportionately with frequency. Thus even though the internal voltages in the machine are very large relative to the battery, the maximum currents out of the machine are effectively limited by the constant ratio of voltage to impedance.
It has been found that if the alternator average rectified output voltage is allowed to increase linearly with speed, and it is optimally loaded, the alternator""s output power can increase proportionately with speed. It has been found that an alternator, at xe2x80x9cdoublexe2x80x9d idle speed, is capable of outputting twice the power if the alternator""s effective load is adjusted so that the output voltage is allowed to increase by a factor of two. As a result, the effective load resistance on the rectified alternator voltage is doubled and the alternator""s average rectified output current remains constant. Thus, the output power of alternators at higher speeds can be significantly greater than the output power implied by the aforementioned characteristic curve which is based upon the alternator being constantly and effectively clamped to a fixed voltage.
Because of the higher power capability at higher speeds, dual voltage implementations are presented. Dual voltage systems allow the classic 14 volt system to be preserved while allowing higher power applications to be implemented at the higher voltage to keep system currents reasonable.
In several embodiments of the alternator system of the present invention, a Lundell style alternator is employed which is augmented by power electronic switching components in the output section. Some of these power switching embodiments may require a scaling of stator turns and wire size to achieve the superior performance at higher speeds.
The alternator system of the present invention provides significantly enhanced power capability above idle as compared with classical Lundell alternator systems using Lundell alternators of the same size that are designed with the same power capability at idle.
In one preferred dual voltage embodiment, a power switch (semi-conductor) connects the alternator output directly to the low voltage load at low engine speeds and when the power switch is xe2x80x9copenedxe2x80x9d at higher speeds, the alternator""s power is channeled to the higher voltage. In a 14- volt/42-volt dual voltage system, this arrangement provides triple the alternator""s power capability at high speeds as compared to a classic un-enhanced alternator system whose output is directed to a single output voltage. This is accomplished without increasing the alternator size or required current capacity and no high frequency semiconductor power modulation is required.
The alternator system of the present invention is particularly applicable to emerging systems with electronic valve actuation as valve power increases rapidly with vehicle speed becoming very large at high speeds. The enhanced system is also applicable to vehicles equipped with a high power electronic windshield de-icing function. In this situation, the vehicle is automatically operated at relatively high idle when the de-icing function is employed to obtain the high power required without increasing the alternators size.
The alternator system of the present invention effects optimum matching of an effective load resistance to the alternator output so as to achieve the theoretical proportional power increase capability with speed. For maximum power output performance, the alternator system effectively maintains an average rectified alternator output voltage of about 60% of the peak internal line-to-line EMF of the alternator which would vary with the speed of the alternator. Even optimized operation over a limited speed range, such as 2:1, can provide significantly more output power at the higher speed and double the alternators capability at very high speeds as compared to a non-enhanced conventional system. A fixed, controlled maximum rectified DC (direct current) alternator output current can effectively provide this maximum power for a given alternator at different operating speeds. In accordance with the present invention, the optimum matching of an effective load resistance to the alternator output, as discussed above in the foregoing description, is achieved with a power switching circuit in the output section of the alternator. Several specific examples of possible organization of these elements and their control implementations are described in the ensuring description.
The enhanced-power alternator system of the present invention can be used for effecting operation of a vehicle at a relatively faster idle when an electronic windshield de-icing function is present and activated. Specifically, the alternator system of the present invention automatically effects a relatively higher idle frequency when the vehicle is started on a very cold day and whenever the vehicle is in park, in neutral or the emergency brake is on and the windshield de-icier function is simultaneously activated.
In accordance with the present invention, the enhanced-power alternator system of the present invention generally comprises an alternator and associated electronic circuitry that is configured to produce maximum power at idle and proportionately increase power at fast idle. In a preferred embodiment, the fast idle speed is selected so that the extra power required for deicing does not require increasing the size of the alternator. Thus, the size of the alternator can be based only upon the non-de-icing power requirements at normal idle.
The electronically power-enhanced alternator system of the present invention meets the higher valve power requirements at high speed without requiring any basic alternator size increase at all. Furthermore, the alternator used in the alternator system of the present invention can operate without increased belt and pulley sizes, as maximum torque capability would remain the same if the alternator used in the alternator system of the present invention and the conventional alternator have the same maximum power capability at idle. A significant advantage of the alternator system of the present invention is that the alternator used therein can maintain the aforementioned maximum torque level at higher speeds.
The alternator system of the present invention is also applicable to other variable field alternator technologies.